README: modify Quick Start for docker build
README: modify Quick Start for docker build

1. Mark menuconfig and clean/distclean as optional steps
   with sub-title because they are not essential steps.
  essential steps : install docker -> clone source code ->
     configure -> build -> program
  optionals : menuconfig and clean

2. Add the program step.
  This step introduces a programming.

3. Modify content of clone and clean tabs.

Signed-off-by: sunghan-chang <c954e25285721d8faa8b1a5c1ae2ef5a35338d32@samsung.com>

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